1. Field of the Invention
The present invention relates to an optical encoder adapted for tracking control in a hard disc apparatus or the like.
2. Description of the Prior Art
Relative to the conventional hard disc apparatus known heretofore, there is an exemplary one disclosed in U.S. Pat. No. 4,396,959 and Reissue 32,075. In such known apparatus, as shown in FIGS. 5 and 6, a hard disc 3 serving as a recording medium is rotated in the direction of an arrow a by a turntable 2 disposed on a chassis 1 used also as a case, and simultaneously a head arm 5 furnished with a flying magnetic head 4 at its fore end is so driven by a voice coil motor 6 as to swing in the directions of arrows b and b' on an arm shaft 7, so that an operation of recording or reproducing the hard disc 3 is performed by means of the magnetic head 4.
An optical encoder 9 employed to execute tracking control in such hard disc apparatus comprises a scale 11 composed of a glass plate and attached to the head arm 5 firmly through a wing 10, a reticle 12 of a glass plate and a light sensitive element 13 such as a photo diode disposed in one lateral lower portion of the scale 11, and a light emitting element 14 such as a light emitting diode disposed in the other lateral upper portion of the scale 11. A sensor 15 is constituted of such reticle 12, light sensitive element 13 and light emitting element 14. And the sensor 15 is anchored onto the chassis 1 by the use of a support 16 composed of synthetic resin or the like.
In the conventional optical encoder 9, as illustrated in FIG. 7, a multiplicity of slit patterns 11a formed in the scale 11 are arrayed radially with respect to the center 7a of the arm shaft 7 in a manner to be arcuate around the center 7a, and four slit patterns 12a each composed of a plurality of slit groups and formed in the reticle 12 are also arrayed radially with respect to the center 7a of the arm shaft 7 in a manner to be arcuate around the center 7a. And four light receiving portions (not shown) of the light sensitive element 13 are disposed under the four slit patterns 12a in the reticle 12.
For simplifying positional alignment of the scale 11 and the reticle 12, it has been customary that the phases of pairs of the slit patterns 12a in the reticle 12, i.e., a total of four phases A, A and B, e,ovs/B/ are arranged linearly on the two sides of the reticle center P (the central position of the reticle 12 in the swing directions b, b' of the scale 11 and on a radial line passing through the center 7a of the arm shaft 7 as illustrated in FIG. 5).
The light irradiated from the light emitting element 14 and passed through the slit patterns 11a in the scale 11 is passed sequentially through the slit patterns 12a in the reticle 12 and then is detected (sensed) by the light receiving portions of the light sensitive element 13. In this stage of the operation, if the scale 11 is in its motion in the direction of the arrow b for example, detection outputs of four phases in the order of B-A-B-A are obtained sequentially as shown in FIG. 8A. And tracking control is executed in accordance with such four-phase detection outputs to obtain a just track position H.sub.1 of the magnetic head 4 relative to the four tracks T on the hard disc 3, as represented by oblique lines in FIG. 8C.
However, there exists a disadvantage in the prior art that thermal off-track is error is prone to occur in case the reticle 12 and the light sensitive element 13 integral therewith are positionally deviated, because of temperature fluctuation from normal temperature to any higher point, with respect to the scale 11 in the direction of y-axis shown in FIGS. 5 through 7.
That is, due to the positional deviation of the reticle 12 in the y-axis direction, the four phases A, and B, B of the slit patterns 12a in the reticle 12 are shifted along the y-axis from the proper positions represented by solid lines in FIG. 7 to the positions represented by dotted lines, whereby one pair of phases A, A on the left side of the reticle center P are varied to have a positional deviation X.sub.1 in the direction of the arrow b', and another pair of phases B, B on the right side of the reticle center P are varied to have a positional deviation X.sub.2 in the direction of the arrow b.
Consequently, regarding the phases of the slit patterns 12a in the reticle 12 with respect to the slit patterns 11a in the scale 11 during the displacement of the scale 11 in the direction of the arrow b, a lead occurs in one pair of phases A, A on the left side of the reticle center P, while a lag occurs in another pair of phases B, B on the right side of the reticle center P. As a result, phase deviations are caused in the entirety of the four detection outputs B, A, B, A in such a manner that, as represented by solid lines in FIG. 8B, the phases B, A and B, A of the detection outputs mutually approach while the phases A, B and A, B mutually recede. Thus, as represented by one-dot chain lines in FIG. 8C, a track pitch error H.sub.2 is induced in the magnetic head 4 with respect to each of the entire four tracks T on the hard disc 3.
For the purpose of solving such a problem, it has been necessary heretofore to perform phase adjustment among the individual phases of the detection outputs by the use of four offset control rheostats and four gain control rheostats for the four phases of the slit patterns 12a in the reticle 12. Accordingly, another problem is existent that a total of eight control rheostats are needed on the light receiving side of the sensor 15.